I've read about one, even with an oracle attached. I've read about it ages ago and have, of course, forgotten the name of the author, the book's title, even the name of the temple's location et cetera. :( I recall that it was a book about ancient Greek oracle sites, and I have no idea if this book can be classified as reliable. Most likely not. *g* Here's what I recall:

The excavations revealed a rectangular room beneath the ground (a hole in the ceiling being the only opening) that was used as a "vessel" for the blood of sacrificial animals. The thick layer of humus soil inside the room pointed to this since it was unlike the soil around. The still visible discolouration of the walls showed the "blood level" inside - two thirds. (The stench must have been abominable.)

This temple was called unique as it was the only known oracle of Hades.

About the temple building I recall nothing at all, except that it had been built built into the mountainside/hillside, from where a system of natural caves could be entered. Excavations revealed a set of flattened gear wheels and a huge cauldron inside the cave system that are suspected to have been part of the 'oracle machine'. The find was made close to (or inside?) a vertical pit within the cave, one side of the pit had been secured by a probably waist-high wall.

I think there has also been found evidence of rooms in small side-caves.

Apparently the shades of the dead could be asked one question at this place. The question had to be delivered to the priests of the temple, along with information about the deceased. The 'client' had then to go through an elaborate 30 day ritual of cleansing; the author suspected this was to give the oracle time enough to dig out information about the deceased in question. The rooms within the cave I mentioned before might have served as rooms for those who wanted to consult the oracle. I also recall something about people averting their faces from the temple as not to draw Hades' attention to themselves. From the excavated gear wheels it was suspected that the oracle priest had been lowered into the pit, and when the time for the 'client' had come to ask his question, he was led to the waist-high wall. Then the oracle priest was lifted up from the pit as if he'd be floating up from the netherworld.